Payload assert : nondeterministic error with timestamps

[Startouf]

DateTime objects are not always converted to the same string in the payload, there is a 1/1000sec difference which causes nondeterministic errors when validating timestamps

Example

Expected JSON payload to have key 'ends_at' == 
"2017-07-16T11:55:12.241+02:00" 
but was 
"2017-07-16T11:55:12.242+02:00"

This failure happens roughly 1/2 of the time, might be a rounding problem.

FYI I am running on

• Ruby 2.3.3p222 (2016-11-21 revision 56859) [x86_64-darwin15]
• rails-5.0.2

[Startouf]

As discussed in Slack, this seems to be related to Ruby's DateTime implementation.
When using Mongoid's type: Time in the models the error is gone.